AC transfer circuits are switching arrangements that permit a load to be transferred from one power source to another independent power source. Their purpose in transferring the load is to assure continued power to the load by disconnecting it from a failed power source and reconnecting it to an active power source. Normally, the load is powered by some primary AC power source such as a commercial power line and when commercial power fails, the load is transferred to a reserve AC power source which normally comprises a battery energized static inverter circuit. When commercial power is restored, the load is reconnected to the commercial AC line. Since the two power signals are not synchronized, i.e., the line and reserve signals are normally not in phase, the reconnection is not permitted until the two signals are in substantial phase agreement in order to avoid large transients when reconnecting to the AC line. Substantial phase coincidence is considered to have occurred when some common feature of the signal such as a zero crossing of the two signals is less than some small time interval apart, otherwise known as within a phase time window. This arrangement does not, however, determine if the two signals are at substantially the same frequency and hence a transfer could occur even if the two signals are not matched in frequency.
A suitable comparator circuit for such an application should define a precise phase and frequency time window which is operative to permit a load transfer only when two drifting signals of only slightly different frequencies are in substantial phase and frequency coincidence.